1. Field of the Invention
This invention relates to a steering wheel in which an impact energy absorber made of sheet metal is disposed in a pad disposed above a boss portion.
2. Description of the Prior Art
As shown in FIG. 1, a conventional impact energy absorber 11 made of sheet metal disposed in a steering wheel 1 is composed of a deformation portion 12 having a section in nearly an inverted U shape provided with an upper wall portion 13 and leg portions 14 and 15 extending downwardly from the front and rear edges of the upper wall portion 13 leg portions 14 and 15 are formed by bending the sheet metal by press working a base portion 16 connected to the lower ends of the leg portions 14 and 15 of the deformation portion 12. The leg portions 14 and 15 of the deformation portion 12 are connected to the base portion 16 such that the lower ends of front and rear leg portions 14 and 15 are extended and bent inside the impact energy absorber 11, respectively, and the bent portions 14a and 15b are connected to the base portion 16 by welding. Further, the upper wall portion 13 of the deformation portion 12 is formed so that the front side is lower by shortening the length of the front leg portion 14. In addition, front and rear leg portions 14 and 15 of the deformation portion 12 are buckled in the middle as at buckling points 14b and 15b so that respective leg portions 14 and 15 buckle easily when an impact force is applied from above of respective leg portions 14 and 15. Reference numeral 3 denotes a boss plate, numeral 4 denotes a spoke core member and numeral 5 denotes a pad (see Japanese Patent Application Laid-Open No. 58-152660).
In the conventional steering wheel 1, when the impact force F is applied from right above to the center of the upper wall portion 13 of the impact energy absorber 11, the upper wall portion 13 first bends in to some extent so as to dent the center thereof. When the upper wall portion 13 bends so as to dent at the center thereof, the front and rear leg portions 14 and 15 are elastically deformed so as to extend outwardly. However, since above said impact force F is big and deforms portion 12 in a short time, the leg portions 14 and 15 buckle either inwardly or outwardly in the vicinity of buckling points 14b and 15b and the upper wall portion 13 approaches the base portion 16 so as to absorb the energy of the impact force F. The reason why the leg portions 14 and 15 extend outwardly or collapse inwardly at the buckling points is that, although bending occurs at the neighborhood of intersections X1 and X2 with leg portions 14 and 15 of the upper wall portion 13 thus producing work hardening, bending also occurs at the buckling points 14 b and 15b thus producing work hardening. Because the bent angles at the buckling points 14a and 15b are shallower and the degree of work hardening is lower than that at intersections X1 and X2, deformation first begins at buckling points 14b and 15b.
In order to make visibility with the conventional steering wheel 1 better, it is necessary to modify the upper wall portion 13 by inclining it more as an impact energy absorber 110 as shown in FIG. 2.
With this impact energy absorber 110, however, a component force Fa of the impact force F working in a direction at a right angle to the upper wall portion 13 will have a great influence when the impact force is applied from right above to the center of the upper wall portion 13.
That is, in this case, the front leg portion 14 and the rear leg portion 15 will be deformed so as to collapse rearwardly without buckling at buckling points 14b and 15b. Instead, the absorber will deform at the intersections of the upper wall portion 13 and legs portions 14 and 15 and at the intersections of bending portions 15a and 14a and leg portions 15 and 14, respectfully.
In the impact energy absorber 110, however, work hardening is produced in the neighborhood of intersections which are bent, and an initial load at the time of deformation is heavy when the front leg portion 14 and the rear leg portion 15 fall to the backside as described previously.
That is, when the impact force F is applied to the center of the upper wall portion 13 from right above, the upper wall portion 13 bends first in some degree so as to dent the center thereof (see the middle figure in FIG. 3). At this time, the neighborhood of respective intersections X1 and X2 bend and work hardening is produced there, and the front and the rear leg portions 14 and 15 are deformed elastically so as to bow outwardly. In this state, inflection points X11, X12, X21 and X22 between a circular arc portion at respective intersections where bending is applied and work hardening is produced and a straight line portion where no work hardening is produced are bent.
Thereafter, the front leg portion 14 and the rear leg portion 15 are deformed in such a manner that they fall rearwardly of the steering wheel with the respective intersections X1, X2, Y1 and Y2 plastic deformed parts (see lower figure in FIG. 3). At this time, the region of the inflection points X12, X21, X22, Y11 and Y12 are also bent practically.
At this time, in the front leg portion 14, the intersection Y1 is applied with bending at an acute angle. As a result, the part Y11 in the region of the intersection Y1 where bending is started is high, and in the moment of the component force Fa of the impact force F acting at a right angle to the upper wall portion 13, the distance l 0 from the working point to the deformation part Y11 is short, so the initial load at the time of deformation of the deformation part Y11 is high.